Transcription

2 Atomization Atomization is the most important production method for metal powders. The process generally consists of three stages: melting atomization (dispersion of the melt into droplets) solidification and cooling. additional processing becomes necessary before the powders attain their desired properties,( reduction of surface oxides, degassing, size classification, etc.)

3 Classification For each of the process stages different methods can be used, resulting in a large number of variants.

4 Melting In this stage the most important criterion is whether melting and melt distribution require a crucible system or not. Crucibles are one of the main sources for contamination of atomized powders. The second criterion is the heating source. Essentially all melting techniques known in metallurgy can be used, e.g., induction, arc, plasma, and electron-beam melting, but some of these may also contribute to contamination, as for example in arc melting.

5 Solidification During the soldification and cooling stage, the cooling rate is the controlling parameter. Cooling rate depends on: the dimensions of the liquid droplets or solid powder particles, the type of heat transfer from the particles to the surrounding medium. The undercooling prior to nucleation and cooling rate are the controlling factors for determining the microstructure of the powder particles, as well as for the dimensions of the atomization unit. Depending on the parameters in each stage various disintegration methods are possible:

7 Gas Atomization Air, nitrogen, argon or helium are used, depending on the requirements determined by the metal to be atomized. Here, melted initial material is collapse with the gas within the chamber. Heat is transfered from the liquid droplets to the surrounding gas. And liquid solidifies. Vertical atomisation unit Atomization can be undertaken either in vertical or horizontal units.

8 Gas Atomization Production of Fine Particles; a high velocity of the atomizing fluid small melt stream (droplet) diameters high density, and low viscosity and surface tension of the melt The nozzle design is an important key to success or failure of a gas atomization system.

9 Gas Atomization The mean particle size of gas atomized powders is in the range of µm. The particle shape is spherical or close to spherical. Irregular particle shapes can only be produced in systems where reactions between the gas and the liquid metal cause the formation of solid surface layers. This is the case, for example, in the air atomization of aluminium.

10 Gas Atomization Melt atomization by air is used in the production of the so-called 'Roheisen Zunder' (RZ) iron powder. This process starts from a cast iron melt. The surface oxides formed during atomization are reduced by the inherent carbon of the cast iron particles during a simple subsequent annealing treatment

11 Gas atomization Inert gas atomisation is applicable for all metallic alloys which can be melted. The main application is for high alloy products such as stainless steel, tool steels, iron, nickel- or cobalt-base superalloy powders, as well as aluminium alloy powders. Limiting factors are the availability of suitable crucible and auxiliary/helpful melting process materials. Powders from refractory metals with high melting temperature and highly reactive materials, such as titanium alloys, are usually produced by other methods.

12 Water atomization Liquid atomization Cyrogenics Liquid atomization Water atomization, Water atomisation is mainly used for the production of iron base powders. Figure shows a scheme for a water atomisation unit. The starting material is melted and metallurgically treated in a separate furnace and then fed into a tundish. The tundish provides a uniformly flowing vertical melt stream, which is disintegrated into droplets in the focal area of an arrangement of several water jets. The impact from the high pressure stream of water leads to disintegration of the flowing metal A high energy input to the water stream is needed. Scheme for a water atomisation unit.

13 Liquid atomization Water atomization Cyrogenics Liquid atomization In water atomization, a high pressure water stream is forced through nozzles to form a disperse phase of droplets which then impact the metal stream. In this method, large quantities of energy are required to supply the water at high pressure. This production method is significant for low and high alloy steels, including stainless steel. Because of oxide formation, water atomization is not likely to be used in the atomization of highly reactive metals such as titanium and the super alloys. In general, water atomized powders are irregular in shape, with rough oxidized surfaces.

14 Liquid atomization Water atomization Cyrogenics Liquid atomization In cyrogenic liquid atomisation,. The Krupp Company (Germany) introduced a novel version of atomization in which the melt is atomised with cryogenic liquid gas (argon or nitrogen) at 200 C. During the process, the pressure of the liquid gas is increased up to 300 bar, while a recooling unit prevents the temperature from rising in spite of compression and prevents the cryogenic liquid from vaporizing instantaneously at the jet opening.

15 In cyrogenic liquid atomisation,. The resulting powder has the following properties: It is much purer than the powder atomized with water and can be compared to the quality of the gas atomized powder. The cooling rate is ten times higher than in gas atomization and almost reaches the quality of water atomization. Particles of 100 µm in diameter, for example, are quenched for approximately 106 K/s. The powders are, as in gas atomization, spherical and have an average size of µm. show satisfactory results in cold forming, while having a good flowability. Gas atomized powders have poor green strength Physical methods Liquid atomization Water atomization: Cyrogenics Liquid atomization

16 Centrifugal atomization The basis of centrifugal atomization is the ejection of molten metal from a rapidly spinning container, plate or disc. The material in the form of a rod electrode is rotated rapidly while being melted at one end by an electric arc. Molten metal spins off the bar and solidifies before hitting the walls of the inert gas filled outer container.

17 Centrifugal atomization The process was developed primarily for the atomization by high purity low oxygen content titanium alloys and superalloys. Powder particles are smooth and spherical with an average diameter of ~200 µm; the size range is µm. Typically, yields run to ~75% for 35 mesh powder.

18 Centrifugal atomization Particle size distribution Accurately controlled rotation of the anode is important so as to obtain in a desired range. as the surface tension of the liquid metal, centrifugal forces (related to rotation speed) to some extent by the aerodynamics of the droplets trajectory through the inert cover gas.

19 Centrifugal atomization Spherical metal powders made by either REP (rotating electrode process) or gas atomization are not well suited for cold pressing into green compacts to be followed by sintering. They are used in more specialized applications where consolidation is achieved by hot isostatic pressing (HIP) or some other high temperature method in which inter particle voids are more readily closed

20 ATOMIZATION Solidification and Cooling The cooling rate the powder particle shape, and microstructure Cooling occurs by heat transfer via radiation heat flow through the medium which is in contact with the melt. The cooling rate during solidification depends on; The melt volume The heat flow Small melt volumes favor rapid cooling. Atomization under vacuum or reduced pressure conditions results in relatively low cooling rates

21 ATOMIZATION The microstructure of the powder particles is determined by nucleation and growth factors. After nucleation has occurred, the relation of the growth rate of the solid from the liquid and the diffusion rate in the liquid determines the microstructure. A large undercooling increases the growth/diffusion ratio Low growth/ diffusion ratios result in dendritic microstructures as a consequence of segregation. With increasing cooling rate, the particles tend to form microcrystalline structures, combined with other non-equilibrium states such as high defect concentrations or metastable phases, which promote sintering effectively. Even non-crystalline (amorphous) structures can be obtained.

22 water atomisation (a), b gas atomisation (c) rotational atomisation (d) Spherical particles result from the solidifying droplets in gas and centrifugal atomization as a result of surface tension. Water atomised powders can be produced with very irregular particle shapes for high green strength requirements, but particles close to spherical can also be produced if good flowability and tap density are required.

25 Powder Characterization Powder characterization and microstructure control in powders The success of any powder metallurgical process depends to a great extent on the complete characterization and control of the metal powders. These properties also influence the behavior of the powder during compaction and sintering, and the composition, structure and properties of the sintered material.

26 Powder Characterization Chemical Composition and Structure The levels of impurity elements in metal powders can be very significant to both the processing and properties of the final product. It is necessary to know whether such elements are present in their elemental form or whether they are present in the form of a chemical compound Practically any metal powder adsorbs significant quantities of gases and water vapour from the atmosphere during storage. The amount of such contamination increases with decreasing particle size and with increasing chemical activity of the surface.

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